Photoinitiator chemicals are those compounds which mediate, facilitate or otherwise cause those reactions which occur during the curing of an adhesive. Curing is a chemical phenomenon which occurs when certain atomic bonds are broken and reformed to create a compositional form which cannot be recast through the application of heat. These adhesives are generally regarded as "thermosetting" and are most useful in the creation of a strong, heat resistant, rigid adhesive bond. Through the application of ultraviolet energy, a photoinitiator constituent facilitates the reformation of chemical bonds in a thermoset adhesive to cure the adhesive.
One application for photoinitiator cured thermoset adhesives is in the assembly of component parts used in data storage and retrieval systems such as CD-ROM technology and hard disk storage technology. The manufacture of hard disk drives involves the use of sundry adhesives spanning numerous applications. Due to the automated nature of the hard drive assembly process, rapid adhesive tack time is of paramount importance. In order to accommodate such process requirements, photosensitive adhesives are often the material of choice. Among this class of adhesives, photosensitive, epoxy-based formulations are often selected due to their rapid fixture times, appreciable dark reaction, and excellent physical properties.
During the manufacture of hard disk drives, adhesives are used for sundry assembly applications. Volatile compounds from the adhesives have been known to deposit on the disk surface resulting in head/disk interface failures. These compounds are typically low molecular weight entities that readily diffuse through the bulk of the adhesive and subsequently effuse into the disk environment. Low molecular weight photoinitiator compounds are exemplified by Crivello et al 4,136,102 and 4,161,478 which each teach onium salt-type initiators of phosphorous, arsenic, and antimony. Crivello teaches in each of these patents that the onium salts can be employed as cationic photoinitiators when used with a variety of organic resins.
Smith et al, U.S. Pat. No. 4,173,476 also teaches salt complexes which may be used as photoinitiators and epoxy compositions. The disclosed compounds are triarylsulfonium complex salts having a discrete molecular weight. Crivello et al, U.S. Pat. No. 4,442,197 also teaches photocurable compositions. The disclosed compositions are described as dialkylphenacyl sulfonium salts or hydroxyaryldialkyl sulfonium salts of a discrete molecular weight. These compounds are polymerizable into epoxy resins through the use of ultraviolet light at a wave length greater than 300 nm.
Chang U.S. Pat. No. 4,197,174 teaches a method for producing bis-[4(diphenylsulfonio)phenyl] sulfide bis-MX.sub.6 initiators which are useful in the cationic polymerization of monomer formulations. Here again, the disclosed compositions are polymerized into epoxy compositions through the application of ultraviolet energy.
Iyer, U.S. Pat. No. 4,400,541 discloses bis-(diphenylsulfoniophenyl)-sulfide bis-chloride salts again of discrete molecular weight which are used as starting materials for the manufacture of other salts which has utility as catalyst for curing ultraviolet activated coating compositions. Angelo et al, U.S. Pat. No. 5,047,568 teaches sulfonium salts having a discrete molecular weight which may be used as photoinitiators for various adhesives.
However, the use of these photoinitiators does not alleviate problems which occur, especially in disk drive assemblies. After assembly, machines carrying disk drive units may be shipped to any number of locations in the world and subsequently stored. Often times, and in most cases, storage conditions may vary radically and defy control by the initial manufacturer. Conditions of high temperature and high humidity are more the rule than the exception in many countries. In such conditions, disk drives have a tendency to film due to many of the chemicals present in the disk drive environment.
A large body of evidence exists implicating various compounds (e.g., silicones and acrylates) in head/disk interface failures. Trial and analysis over time has illustrated that the composition of the film which forms on disk drives is corrosion and has a high concentration of sulphur. The use of sulfonium salts such as those found disclosed in the patents cited above leads to low molecular weight volatile species which outgas and deposit upon the disk drive to form these corrosive films.
Therefore, there is a need in the art for photoinitiator compounds and polymer systems which will effectively cure adhesives to a desired bond and which will not outgas low molecular weight constituents which, in turn, will corrode disk drive mechanisms or other component parts.